The use of filters to remove selected frequencies from AC power is well known. For example, a variable speed, constant frequency (VSCF) power generating system includes filters for removing harmonics from the system output power. In such a system, a DC link filter removes a ripple component from DC power provided on a DC link by a rectifier. Also, AC power produced by a controlled static inverter coupled to the DC link is filtered by filtering components to remove harmonics present in the inverter output so that a nearly sinusoidal output can be obtained.
Typically, inverters which are used in such VSCF systems are of the three-phase, four-wire type wherein three phase output voltages and a neutral are produced. The output filter in such a system is connected in a wye configuration wherein impedances are connected between each phase output and the neutral.
It has been found that the phase-to-phase displacement of each harmonic in the AC power produced by the inverter varies with the order of the harmonic. The variation in phase-to-phase displacement can render wye-connected or delta-connected filters inefficient or even ineffective to remove certain harmonics. This can in turn lead to the requirement for additional filtering components and/or larger and heavier components in the output filter.
VeNard II, U.S. Pat. No. 3,746,963, discloses the use of a DC filter coupled on a DC link between a rectifier output and an inverter input. The filter comprises a first capacitor coupled in parallel with a series combination of an inductor and a second capacitor. The filter is in turn coupled across two conductors of the DC link. The filter is tuned to a frequency equal to twice the output frequency of the inverter.
Pickup, et al., U.S. Pat. No. 3,461,372, discloses a DC to AC power converter comprising a transistor which is operated by a square wave input and a plurality of series resonant and parallel resonant filters coupled to the collector of the transistor. Even harmonics in the power produced at the collector of the transistor are attenuated by the series resonant filters while lower odd harmonics are blocked by the parallel resonant filters. A sinusoidal output is thereby produced at a frequency equal to the fundamental frequency of the square wave input to the transistor.
Hucker, PCT WO 85/01844 (assigned to the assignee of the present application) discloses a sine-wave resonant converter which produces a sine-wave output from DC power provided by a DC power supply. A series resonant circuit is coupled across the DC power supply and is tuned to twice the frequency of the inverter output to provide a low impedance path for AC current flowing to the power supply.